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The facts about the energy efficiency of windows

What you should know about window technology.

Much of the heat produced by our heating systems, and the cool air produced by our air conditioning systems, is lost through the windows of our home. This affects us economically as we pay higher energy bills. We burn more oil unnecessarily and consume more electricity. At a national and global level, we spend scarce resources quicker and create more carbon emissions than we should. Hence the need for energy efficient windows.

Traditionally windows were a source of drafts and lost heat through badly fitting frames and thin panes of glass. Nowadays, window construction is an advanced technology that, if correctly employed, can save significant amounts of money over the lifespan of the window.

Choose the right sort of efficiency.

New windows are labeled according to their energy efficiency, and this may be important for your local building codes. You will need to take account of the type of climate in which you live when you decide which replacement window is right for you. There are some windows that are more efficient at keeping out the cold and retaining heat than others. Some are better at keeping the heat out and retaining cool air. The latter is more suitable for the southern US, while the former is more appropriate for the north. There are also models available that offer a balanced solution to the two extremes.

Energy efficiency terms.

The following are the common terms used to describe the efficiency of any particular window.

The most important factor in determining a window's energy efficiency is the U-value. It is the measure the heat transfer and infiltration rates of the window. A low U-value suggests that the window is more resistant to hear flow. (There also exists a less precise measure of the conductive heat loss of a window known as the R-Value. While a low value U-Value expresses more resistance, the same is expressed by a high R-Value).

The Solar Heat Gain Coefficient or SHGC is a measure of the how heat radiation from the sun is admitted through a window.

Another important factor is Air Leakage or AL. This is a measure of the ease with which air passes from one side of the window to the other through poor sealing around the window unit. The measure is expressed in terms of the number of cubic feet of air that can be lost through 1 sq ft. of window area in a minute.

Visible Transmittance or VT is a measure of the amount of sunlight that passes through a window. The amount of transmittance depends on the number of panes - VT is lower for a double glazed window than for a single pane - and the presence of tinting or mirroring which will reduce the VT value.

The right windows for the right climate

Cold climates

The ideal window for a cold climate will have a low U-value of say 0.35 or lower, meaning that less heat will escape from the house. In this type of climate you will want the sun to warm the house all it can, so you will aim to find a window that has a high SHGC of 0.75 or more. You will want to avoid air leakage so the AL factor should be less than 0.30. You will probably be interested in a window with a high VT rating of say 0.7, indicating that daylight will pass easily into your home.

Hot climates

A U-value of 0.75 or higher will be more appropriate and compatible with the use of cooling systems. Solar heat gain will be something best avoided and therefore a window with a SHGC value of less than 0.40 will be desirable. As with a cold climate you will want a low AL factor, so the cool air created by your air conditioning is not easily lost. Depending on your daylighting needs, you will either want to have a window with a low VT value, which is a function of achieving a low SHGC through tinted or reflective glass, or you will look for some of the new spectrally selective glass. These glass types maintain a high VT value while blocking or re-radiating infrared energy from the sun, so reducing solar heat gain. This type of product reduces the cost of artificial lighting.

Mixed climates

If your home infrequently or never uses an air conditioning system, you should choose windows with a lower U-value of say 0.40 and a mid-range SHGC value of 0.50.

More information is available on windows and their energy efficiency ratings is available from the National Fenestration Rating Council, which rates the windows of their member manufacturers.

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